Analysis of progress of oxidation reaction during oxygen-alkali treatment of lignin I: method and its application to lignin oxidation

A new method is applied to evaluate the progress of the oxidation reaction of lignin during oxygen-alkali treatment. This method employs the differenc e in permanganate consumption of the sample before and after the oxygen-alk ali treatment as an indication for the lignin oxidation. When kraft lignin and residual lignin isolated from unbleached softwood kraft pulp were subje cted to oxygenalkali treatment up to 6000 min, the progress of the oxidatio n expressed by this method was separated into clearly distinguished three p hases. During the first and second phases, the progress of oxidation was we ll correlated to the loss of methoxyl group and to the decrease in the yiel d of nitrobenzene oxidation products. The addition of Mn2+ to the oxygen-al kali treatment depressed oxidation during the second phase partly and that during the third phase almost completely. Calculations based on the change in the permanganate consumption revealed that the oxidation during the firs t phase corresponded to 4.2 electrons abstracted from one lignin structural unit on average. The oxidation process by oxygen-alkali treatment was hypo thetically attributed to the direct reaction between molecular oxygen and t he phenolic unit of lignin, which mainly took place during the first phase, and to the autooxidation-type oxidation during the second and third phases .